1. Field of the Invention
This invention relates to a lens (a so-called f.theta. lens) in which a beam deflected by a laser beam deflector is focused on a scanning surface to scan it at uniform speed.
2. Description of the Prior Art
Known f.theta. lenses used for measuring apparatus, examination apparatus and the like which use a laser printer and a laser beam deflector include various kinds of lenses such as a single lens, a two-group lens, etc. However, in case of a wide angle whose deflection angle is above .+-.25.degree., many of lenses which are relatively simple in construction have insufficient curvature of field and f.theta. characteristic.
For example, according to an embodiment disclosed in Japanese Patent Application Laid-Open No. 137,631/78, the curvature of field is 5 to 10 mm in case of f=300 mm, and a deviation of the f.theta. characteristic is relatively large, i.e., approximately 0.5%. In a single f.theta. lens disclosed in Japanese Patent Application Laid-Open No. 5,706/83, if a deflection half-angle exceeds 25.degree., both the curvature of field and f.theta. characteristic have a problem.
This invention is intended to provide an f.theta. lens which comprises a small number of lenses, and in which even in a case where a deflection half-angle is 30.degree., it has a small curvature of field and an excellent f.theta. characteristic.